Carbon nanotubes are tube-like molecules 1 nm to several ten nm in diameter and 0.1 to several μm in length, which are classified into the mono-layer type and the multi-layer type depending on the number of layers of the walls constituting their hollow structures. These molecules are expected as promising mechanical/electronic materials because they are extremely thin and long, high in mechanical strength, excellent in electrical properties, and high in structure stability. At present, they are mainly used as materials for engineering purposes, and are now being put into practical use as highly efficient electron radiation sources and probes for atomic force microscopes. Although still in the research stage, application of carbon nanotubes to such as microactuators or gas sensors is now being studied. On the other hand, in the field of biochemistry, their application to probes for gene transfer or micromanipulators is being studied. However, in any of their usages, electron microscopes are generally used for mechanical handling of carbon nanotubes because of difficulty in their optical recognition since their diameters are below the limit of optical resolution. Further, methods of modification of carbon nanotubes currently known are, for example, a method in which the carboxyl group generated at the site of cleavage of the structure is used; a method using high temperature fluorine; and a method in which defects are generated by sonication in polymethyl methacrylate.
Carbon nanotubes have such properties that they are extremely thin, excellent in structure stability, high in mechanical strength, and good in electrical properties. However, on the other hand, their properties give rise to the following problems: it is difficult to optically recognize them because their diameters are below the limit of optical resolution; it is difficult to process them because they are high in mechanical strength; it is difficult to introduce into them groups to be reacted with biologically active substances because they are high in structure stability; and it is necessary to insulate them when they are used as signal cables. In particular, in the application of them to organisms where operations under microscopic observation are indispensable, their poor optical recognition property is a big drawback.
It is an object of the present invention to solve the above-described problems of the relevant technology and to provide novel carbon nanotubes which have overcome the conventional drawbacks.